1. Field of the Invention
The present invention relates to a device for detecting each of the pulses of a pulse train among noise.
In some applications, such as radio-navigation aid systems of the DME (Distance Measuring Equipment) type, it is necessary to detect pulse trains, the train possibly comprising only two pulses as in the case of DME and, more precisely, to identify the pulses so as to detect the spacing thereof, which constitutes data.
2. Description of the Prior Art
Different devices are known for this purpose, particularly within DMEs where such detection is particularly critical. It will be recalled that in a DME the pulses are transmitted in pairs and at reception the spacing thereof is determined, the arrival time of each pulse being determined on its rising front, at mid height.
A known device for achieving such detection is shown in FIG. 1. In this FIG., a pulse pair E forming the input signal, is applied on the one hand to a peak detector D which defines the half height of the pulse, that is to say V.sub.c /2 where V.sub.c is the peak value, and on the other hand to a delay line R.sub.o. The signal coming from elements D and R.sub.o, designated respectively V.sub.c /2 and E.sub.R, are applied to a comparator C.sub.o. The purpose of the delay line R.sub.o is to delay the input pulses E by a time .tau..sub.o so that they arrive at the comparator C.sub.o at the same time as (or after) the detected value V.sub.c /2.
The input signal E is shown in FIG. 2a as a function of time. The two pulses have substantially the same peak value V.sub.c and their spacing T is reckoned as the time interval separating the rising front of each of the pulses, at mid height (V.sub.c /2). FIG. 2b shows, still as a function of time, the signal E.sub.R which is identical to the signal E but delayed with respect thereto by a time .tau..sub.o. When the first delayed pulse (E.sub.R) arrives at comparator C, this latter compares its rising front with V.sub.c /2 and delivers a signal (rectangular pulse) at the time when this front reaches the value V.sub.c /2. The signal delivered by comparator C, designated S, is shown as a function of time in FIG. 2c and the part of the signal S corresponding to the first pulse of the signal E.sub.R is referenced 1. When the second pulse forming the delayed signal E.sub.R reaches comparator C in its turn, this latter operates in the same way and delivers a second rectangular pulse referenced 2 in FIG. 2c. Between the rising fronts of pulses 1 and 2 of the output signals S we find the spacing T which separated the two input pulses E.
It is apparent that the operation of such a device requires the amplitude of the input signal E between two pulses to drop below the value V.sub.c /2. In the opposite case, the comparator C is unable to identify the second pulse. However, this condition concerning the amplitude of the signal E may not be fulfilled when noise is superimposed on the pulse train. For example, when a DME is operating in an environment containing obstacles reflecting the radioelectric waves, these obstacles reflect with a certain delay DME pulses which are then superimposed on the direct propagation signals and form a composite input signal, whose amplitude may be greater than V.sub.c /2 between the two pulses. Thus, the efficiency and accuracy of the whole of the DME is decreased : the number of valid measurements is reduced and in some cases is zero.